The present invention relates to a device for inducing slack in a wrapping film, associated with the conveyors by which packs of rolls are directed through a non-continuous packaging machine. In a packaging machine of the type in question, traditional packs of two or four rolls are wrapped in a covering of transparent plastic film using a process that involves passage through a succession of stations comprising an elevator by which the rolls are received in pairs, in contact one with another and horizontally disposed, and carried upward in this same configuration to the point of entering a first wrapping station beneath which a sheet of film is extended flat in readiness to be fashioned into a wrapping around the pack.
The ascending rolls are first partially enveloped from above, as a result both of investing the sheet and of the reaction afforded by a pair of vertical restraints between which the rolls remain lodged when the elevator returns to the lowered position for a successive cycle. The stations next in line are equipped with understroking horizontal folding or flattening devices, from which the pack emerges with the sheet of wrapping film passed fully around the rolls and the two flattened edges overlapping. The vertical restraints are embodied as projecting elements rigidly associated with a pair of looped chains forming a continuous conveyor, the distance between the centres of the restraints establishing the space afforded to the rolls, and the indexing step of the chains determining the distance covered by the packs when transferred from one wrapping station to the next.
The wrapping operations having been effected, all along a predetermined feed axis, the pack of rolls is positioned at a heat seal station where the overlapping edges of the sheet of film are secured; the station in question consists substantially in a roller occupying a position below the feed axis and of axial dimensions corresponding to those of the sheet of film, rotatable about its own axis in a direction concurrent to that followed by the pack and furnished with a heated sector (heat generated from an external source by Joule effect) by which the temperature of the overlapping edges of the sheet of film is raised to the point of softening and thus enabling a heat seal, preferably at points clearly defined by a serrated profile formed on the sector. In successive steps, the end folds of the wrapping are flattened and secured in conventional manner.
The main drawback betrayed by a pack emerging from the stations in question is that of the excessive peripheral tension in the wrapping. In the process of being enveloped by the plastic film, in effect, the paired rolls are pulled together and forced out of shape over the areas breasted in mutual contact; such deformation is caused mainly by the excessive pressure exerted by the horizontal folding devices, in combination with the pressure exerted from each side by the conveyor restraints (necessary in order to support the pack during its passage through the various wrapping steps), and results in a finished package of substandard appearance.
Accordingly, the object of the present invention is to overcome the drawback in question by providing a device for inducing slack in a wrapping sheet that is simple, efficient and precise in operation, and constructed in a way such as to interfere neither with the structure of conventional work stations nor with the assembly, wrapping and heat seal tempos typical of existing automatic machines.